This invention relates to a channel subassembly for a plasma addressed liquid crystal (PALC) display panel.
U.S. Pat. No. 5,077,553 discloses apparatus for addressing data storage elements. A practical implementation of the apparatus shown in U.S. Pat. No. 5,077,553 is illustrated schematically in FIG. 3 of the accompanying drawings.
The display panel shown in FIG. 3 comprises, in sequence from below, a polarizer 2, a channel member 4, a cover sheet 6 (commonly known as a microsheet), a layer 10 of electro-optic material, an array of parallel transparent data drive electrodes (only one of which, designated 12, can be seen in the view shown in FIG. 3), an upper substrate 14 carrying the data drive electrodes, and an upper polarizer 16. The channel member 4 is typically made of glass and is formed with multiple parallel channels 20 in its upper main face. The channels 20, which are separated by ribs 22, are filled with an ionizable gas, such as helium. An anode 24 and a cathode 26 are provided in each of the channels 20. The channels 20 are orthogonal to the data drive electrodes and the region where a data drive electrode crosses a channel (when viewed perpendicularly to the panel) forms a discrete panel element 28. Each panel element can be considered to include elements of the layer 10 and the upper and lower polarizers 2 and 16. In the case of a color display panel, the panel elements include color filters (not shown) between the layer 10 and the upper substrate 14. The region of the upper surface of the display panel that bounds the panel element constitutes a single pixel 30 of the display panel.
It will be convenient in the following description to refer in general to a given data drive electrode 12.sub.i and a given channel 20.sub.j and to the panel element 28.sub.i,j at which the data drive electrode 12.sub.i and the channel 20.sub.j cross.
The anodes in the several channels may be connected together by a rail (not shown) that is held at ground potential. When a suitable negative voltage is applied to the cathode in the channel 20.sub.j, the gas in that channel forms a plasma which provides a conductive strip at the lower surface of the cover sheet 6. The element of the data drive electrode 12.sub.i and the area element of the conductive strip that is within the panel element 28.sub.i,j form respective electrodes of a parallel plate capacitor; and the electrode at the lower surface of the cover sheet is held near ground potential by the plasma.
If the data drive electrode 12.sub.i is at ground potential when the plasma is created in the channel 20.sub.j, the element capacitor in the panel element 28.sub.i,j is not charged. There is no significant electric field in the volume element 10.sub.i,j of electro-optic material and the panel element 28.sub.i,j is considered to be off. However, if the data drive electrode 12.sub.i is at a substantial positive potential when the plasma is created in the channel 20.sub.j, the element capacitor is charged and there is a substantial electric field in that volume element of electro-optic material and the panel element 28.sub.i,j is considered to be on.
When the plasma in the channel 20.sub.j is extinguished, the conductive strip is destroyed but on principles of conservation the lower surface of the cover sheet remains at a constant potential relative to the data drive electrode until a plasma is again created in the channel 20.sub.j and the resulting conductive strip is connected to ground. Thus, the element 10.sub.i,j of electro-optic material remains in the same state while there is no plasma in the channel 20.sub.j, even though the potential of the data drive electrode 12.sub.i might not remain the same.
It will be assumed in the following description, without intending to limit the scope of the claims, that the lower polarizer 2 is a linear polarizer and that its plane of polarization can be arbitrarily designated as being at 0.degree. relative to a reference plane, that the upper polarizer 16 is a linear polarizer having its plane of polarization at 90.degree., and that the electro-optic material is a twisted nematic liquid crystal material that rotates the plane of polarization of linearly polarized light passing therethrough by an angle that is a function of the electric field in the liquid crystal material. When the panel element is off, the angle of rotation is 90.degree.; and when the panel element is on, the angle of rotation is zero.
The panel is illuminated from the underside by an extended light source (not shown) that emits unpolarized white light. A rear glass diffuser 18 having a scattering surface may be provided between the light source and the panel in order to provide uniform illumination of the panel. The light from the source is linearly polarized at 0.degree. by the lower polarizer 2 and passes sequentially through the channel member 4, the channels 20, the cover sheet 6, and a volume element of the liquid crystal material toward the upper polarizer 16 and a viewer 26. If a given panel element is off, the plane of polarization of linearly polarized light passing through the volume element of liquid crystal material is rotated through 90.degree., and therefore the plane of polarization of light incident on the upper polarizer element is at 90.degree.. The light is passed by the upper polarizer element and the pixel is illuminated. If, on the other hand, the panel element is on, the plane of polarization of the linearly polarized light is not changed on passing through the volume element of liquid crystal material. The plane of polarization of light incident on the upper polarizer element is at 0.degree. and therefore the light is blocked by the upper polarizer element and the pixel is dark. If the electric field in the volume element of liquid crystal material is intermediate the values associated with the panel element being off and on, light is passed by the upper polarizer element with an intensity that depends on the electric field, allowing a gray scale to be displayed.
When the PALC display panel disclosed in U.S. Pat. No. 5,077,553 is used as a raster scan display panel for displaying an NTSC video signal, the panel is oriented so that the channels extend horizontally and the data drive electrodes extend vertically. The first active line of a frame of the video signal is sampled. A negative-going strobe pulse is applied to the cathode in the first channel to establish a plasma in the first channel, and the data drive electrodes are driven to voltage levels that depend on the respective sample values. In each panel element along the first channel, an electric field that establishes the state of the panel element is created between the data drive electrode and the conductive strip at the lower surface of the cover sheet. The strobe pulse is removed, and the plasma is extinguished, but the electric field persists, maintaining the state of the panel element until the first channel is again addressed, on the next frame of the video signal. This sequence of operations is repeated in order for the remaining active lines of the frame and the remaining channels of the display panel.
FIG. 4A illustrates schematically the situation during line j of a frame of the video signal, when there is a plasma in the channel 20.sub.j and the three adjacent data drive electrodes 12.sub.i-1, 12.sub.i and 12.sub.i+1 are all at the same positive voltage V.sub.+.
The electric field vector is primarily perpendicular to the lower surface of the cover sheet throughout the volume element 10.sub.i,j as shown by the electric field lines, and the electric field in the volume element 10.sub.i,j is substantially uniform. The charge density over the lower surface of the cover sheet is substantially uniform within the panel element 28.sub.i,j. When the plasma in the channel 20.sub.j is extinguished, the charge density over the area element of the lower surface of the cover sheet is frozen and the field in the volume element 10.sub.i,j remains the same until line j of the next frame of the video signal.
FIG. 4B illustrates the same situation as FIG. 4A except that the data drive electrodes 12.sub.i-1 and 12.sub.i+1 are not at the same potential V.sub.+ as the electrode 12.sub.i but are instead at ground potential. In this case, it is desirable that the field in the volume element 10.sub.i,j should be uniform and equal to the value in the case of FIG. 4A and that the electric field in the volume elements 10.sub.i-1 and 10.sub.i+1,j should be uniform and equal to zero. However, this would not in fact be the case because there can be significant fringing fields between the electrode 12i and the electrodes 12.sub.i-1 and 12.sub.i+1, as indicated in FIG. 4B by the electric field lines linking the electrode 12.sub.i with the electrodes 12.sub.i-1 and 12.sub.i+1. Consequently, the state of the data drive electrode 12.sub.i is influenced by the state of the adjacent data drive electrodes, i.e. there is crosstalk between the data drive electrodes. As a result of the crosstalk, illumination of the pixels is non-uniform, and even if the non-uniformity in illumination of the pixels were tolerable, the contrast ratio would be impaired.
It is conventional to assemble a display panel of the kind shown in FIG. 3 by forming a channel subassembly, including the channel member and the cover sheet, forming an upper substrate assembly, including the upper substrate, the data drive electrodes and the layer of electro-optic material, and attaching the upper substrate assembly to the channel subassembly. In manufacture of the channel subassembly, the cover sheet is placed over the upper surface of the channel member and is sealed to the channel member around the periphery thereof.